It is well known to provide electronic article surveillance systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with an electromagnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "interrogation zone" the presence of the marker is detected and an alarm is generated. Some markers of this type are intended to be removed at the checkout counter upon payment for the merchandise. Other types of markers remain attached to the merchandise but are deactivated upon checkout by a deactivation device which changes a magnetic characteristic of the marker so that the marker will no longer be detectable at the interrogation zone.
A known type of EAS system employs magnetomechanical markers that include an "active" magnetostrictive element, and a biasing or "control" element which is a magnet that provides a bias field. An example of this type of marker is shown in FIG. 1 and generally indicated by reference numeral 20. The marker 20 includes an active element 22, a rigid housing 24, and a biasing element 26. The components making up the marker 20 are assembled so that the magnetostrictive strip 22 rests within a recess 28 of the housing 24, and the biasing element 26 is held in the housing 24 so as to form a cover for the recess 28.
As disclosed in U.S. Pat. No. 4,510,489, issued to Anderson et al., the active element 22 is formed such that the active element 12 has a natural resonant frequency at which the active element 22 mechanically resonates when exposed to an alternating electromagnetic field at the resonant frequency. Typically, when the marker 20 is assembled, the bias element 26 is in an unmagnetized condition, and the marker 20 is subsequently exposed to a magnetic field in such a manner that the biasing element 26 is magnetized to saturation, in order to provide the requisite bias field to cause the active element to have the desired resonant frequency. Magnetizing the bias element 26 places the marker 20 in an activated condition, so that marker 20 will interact with, and be detected upon exposure to, an interrogation signal generated at or near the resonant frequency of the active element.
The representation of the marker 20 in FIG. 1 is somewhat simplified, and should be understood as indicative of any one of a number of conventional forms in which magnetomechanical markers are actually manufactured. For example, the housing 24 typically includes a top wall (not shown) which intervenes between the active element 22 and the biasing element 26 to prevent the element 22 from being "clamped" by magnetic attraction to the element 26.
Deactivation of magnetomechanical markers is typically performed by degaussing the biasing element so that the resonant frequency of the active element is substantially shifted from the frequency of the interrogation signal. After the biasing element is degaussed, the active element does not respond to the interrogation signal so as to produce a signal having sufficient amplitude to be detected by detection circuitry.
It is customary to manufacture magnetomechanical markers in large batches, and then to activate the markers and ship them in large quantities (hundreds or thousands) to customers such as retailers or manufacturers, who in turn apply the markers to items to be protected from theft. According to a conventional technique for activating the markers, a two-dimensional array of markers is adhered to a release sheet and then the sheet is placed in a pulse coil magnetizer which applies a magnetic field to the markers so that all of the bias elements thereof are magnetized to saturation. Sheets with markers carried thereon are placed one by one in the pulse coil magnetizer to activate the markers and then are stacked in a box for storage and/or shipment to a customer. The conventional process is carried out so that in the resulting stacks of sheets, the respective longest dimensions of all of the markers are arranged parallel to each other, the bias elements 26 of the markers are magnetized along the length thereof, and the north pole of each of the magnetized bias elements 26 is oriented in the same direction in all of the markers. Typically, each sheet carries 50 to 100 markers or more, and about 50 to 100 sheets are contained in each box, so that some 2,000 to 5,000 markers or more are packed together in the box in close proximity to each other.
FIG. 2 schematically illustrates a top view of a box 30 containing sheets of markers which have been activated according to the conventional technique. The arrow 32 in FIG. 2 indicates the common direction of orientation of the north poles of the magnetized bias elements of the markers in the box 30. The aggregation of the magnetized bias elements 26 in the box 30, all having north poles oriented in the same direction, forms a substantial magnetic field proximate to the box 30, as indicated by flux lines 34 in FIG. 2. It will be seen that the flux lines 34 exit from the "north" end 36 of the box 30 and then loop back toward the "south" end 38 of the box 30. A representative marker 20, located at the top and toward the edge of the stack of markers within the box 30, is shown in FIG. 2.
A potential problem, not previously recognized in the prior art, has been noted by the inventors of the present invention. The magnetic field formed by the accumulated markers is experienced by the marker 20 as a "leakage" field oriented in a direction indicated by arrow 40, i.e., in a direction such that the leakage field tends to demagnetize the bias element of the marker 20 if the field is sufficiently strong. If the leakage field is strong enough to demagnetize the bias element 26 of the marker 20, then the marker 20 is placed, unintentionally, in a deactivated condition which causes the marker not to be detectable by the EAS detection equipment to be used with the marker.
According to another conventional practice, after the magnetizing field is applied to the sheets of markers, the sheets are cut into strips, and the strips are spliced end-to-end to form a long strip which carries a single column of markers, with the markers oriented transversely to the length of the strip. The long strips are then rolled to form a roll of markers on the release sheet. This practice again produces a large aggregation of markers, all of which have their bias elements magnetized with a north pole oriented in the same direction, thereby producing the same sort of leakage field illustrated in FIG. 2.
It has been customary to form the bias element 26 from a semi-hard magnetic material having a coercivity of 60 Oe or greater. Since the leakage fields generated by the accumulations of markers that have typically been produced do not exceed about 35 to 45 Oe, inadvertent deactivation of markers located at the edges of a stack or roll of markers has not proven to be a concern.
However, recently developed techniques, such as those disclosed in U.S. Pat. Nos. 5,495,230 and 5,469,140 (commonly assigned with this application) have made it practical to reduce the thickness of the marker housing 24. This, in turn, has led to denser packing of the markers in stacks of sheets or in rolls, and a potential increase in the strength of the leakage fields. Thus, there is an increased risk of inadvertent deactivation by "leakage" field demagnetization of the conventional bias element.
Furthermore, in patent application Ser. No. 08/697,629, filed Aug. 28, 1996 and having a common inventor and common assignee with this application, it has been proposed to form magnetomechanical markers with bias elements having substantially lower coercivities than conventional bias elements. For example, the '629 application discloses a bias element formed by heat-treating an alloy designated as Metglas 2605 SB1, which is commercially available from Allied Signal Inc. After the treatment disclosed in the '629 application, the material has a coercivity of about 19 Oe. Markers formed with bias elements of the treated SB1 material can be much more easily deactivated, when deactivation is desired, but also carry an increase risk of unintentional deactivation due to the leakage field produced by stacks or rolls of markers.